It is desirable to be able to provide a logarithmic transfer function within certain applications. For example, it is desirable to be able to provide a logarithmic transfer function with an automatic gain control loop because having a logarithmic or exponential gain characteristic "linearizes" equations which are used to calculate bandwidth. In other words, the loop bandwidth is made constant with respect to gain. As a result, it is easier to compensate the loop to guarantee stability.
Unfortunately, it is difficult to provide a logarithmic transfer function in a complementary metal oxide semiconductor (CMOS) device because these devices have a square law characteristic. In contrast, a bipolar device, for example, has a current-to-input voltage relationship which is exponential. While a CMOS device will have a current-to-input voltage relationship which is exponential at very low currents, this is not a viable option for circuitry which requires a bandwidth above the 1 MHZ range.
The present invention is directed to provide a transfer function which is generally logarithmic and which can be employed in a CMOS device such as in a programmable gain amplifier.